1. Field of the Invention
The invention relates to a marking device for encoding metallic workpieces with two-dimensional matrix codes in which the information is present in the form of recessed embossed dots in a square or rectangular arrangement. The presence or lack of these embossed dots at the respective grid points represents the binary encoded information.
2. The Prior Art
To read back the information without error, the precision in placing the embossed dots is of high importance. The precise shape, size and depth of the dots are critical quality features. This is directly connected to the type of reading technology for such embossed or punched encodings, respectively, by means of CCD cameras. Illumination from the top or the side must create a contrast between light and dark from the respective recess by means of corresponding reflections, which is much more difficult than with printed black and white surfaces located on one level, for which the code was originally developed. A deviating shape or size of the individual recesses can easily cause (or undesirably not cause) a reflection which can lead to an undesired distortion of information. In the aerospace industry, requirements are even stricter for critical components under high load; these requirements aim at avoiding the reduction of mechanical stability due to the “notch effect”.
In order to achieve the required precision, the striking tool, normally embodied as a hard metal needle, must strike the metallic workpiece, on the one hand, very rapidly, but on the other hand, with precisely defined and reproducible energy. Many conditions must be taken into account as counteracting the desired precision. In case of an electric drive, for instance, the temperature of the copper coil of the electromagnet can increase during operation, reducing current flow and thus the power consumption of the electromagnet. During longer standstill periods of the marking device, the striking tool which is formed as a magnet keeper, or connected to or operatively connected with a magnet keeper, sticks so that the impact energy at the first dot is reduced. In principle, a striking movement which is too slow causes an oval distortion of the recess when the impact unit moves on during encoding. On the other hand, an impact speed which is too fast leads to a great variation in impact depth, since even minimum differences, e.g. due to overlaid mechanical oscillations in the striking mechanism, lead to slightly different energy outputs of the impact system during the formation of the recess. Furthermore, the material properties of the workpiece also influence the formation of the recess. Finally, mechanical tolerances also lead to errors, if they cause the movement of the magnet keeper to exceed the magnetically substantially linear range.
In known arrangements, the current is only intended to be switched on and off for the electromagnet. Clamping diodes or other overvoltage protection equipment are used for protection against overvoltage, when the electromagnet is switched off, as an inductive load. Bias resistors before the electromagnet for inducing a faster rise or drop of current in the magnet coil by increasing the time constant are also known. In these simple systems, in addition to one-time dimensioning, only the time of disconnecting can be varied after the current is switched on, whereas the entire time course of the working movement results exclusively from dimensioning and the prevailing boundary conditions. With such systems, the required precision cannot be attained.
In controlling solenoid valves, on the one hand, it is well-known to switch back to a lower holding current after the high turn-on current, which is first required for a fast movement. This switchover, however, does not take place until after switching of the valve, i.e. after the movement of the valve member, and is intended first to save energy and secondly to reduce heating of the solenoid valve.